Ben Jones and colleagues from the University of Aberdeen in Scotland conducted a study in which they showed 28 men and 28 women pairs of male faces and asked them to rate their attractiveness.  The photos were pre-rated by 40 women and were rated equal in attractiveness. 

After the initial rating, the same men and women were presented with the same photos, however, this time the pairs of photos included a female face in profile.  The female was positioned to be looking (smiling or not smiling) at one of the men.  What were their results?

Women found the men who were being smiled at suddenly more attractive, while men who apparently elicited no such smiling approval were pronounced less attractive.

Men, meanwhile, behaved in a strikingly different manner. They rated men who had been smiled at as less attractive. ”Within-sex competition promotes negative attitudes towards men who are the target of positive social interest from women,” the researchers conclude.